The present invention relates, in general, to the detection and characterization of defects in metal structures and, in particular, to a method for determining the structural integrity of multi-layer structures such as an aircraft fuselage lap splice.
Corrosion is a major problem that can compromise the structural integrity of equipment in many diverse industries such as in pipelines (gas or oil) or in the aircraft industry. In the aircraft industry, this situation is a primary concern to the engineering authority responsible for aircraft airworthiness. One structure closely scrutinized in the aircraft industry is the fuselage lap splice. This metallic multi-layer structure has a design such that a crevice exists where conditions are favourable for corrosion. Corrosion in a lap splice could, ultimately, lead to a structural failure of the fuselage.
Visual inspection is one method for detecting corrosion in multi-layer structures such as in aircraft fuselage lap splices. This technique is based on the principle that, when corrosion takes place between the layers, the metal lost to corrosion results in a product that forces the plates apart and causes surface distortion. The visual inspection, however, does not provide a fool proof indication that the deformation is actually due to corrosion. Such distortion may exist, for example, as a result of poor quality control during manufacturing or from a previous repair. Ultrasonic techniques have also been used to detect corrosion in metal pipes such as the techniques described by David Wang in Canadian Patent Application 2,258,439. The detection of defects, other than those in the first layer, using ultrasonic techniques requires a mechanical bond between plates. The absence of a bond will preclude detection in second, third or fourth layer defects.
Another method for detecting corrosion in metal 10 structures is the use of a low-frequency eddy current inspection method. A low-frequency eddy current inspection technique uses a coil to induce eddy currents in a test object. The induced eddy currents produce a time-varying magnetic field which can be measured by magnetic flux sensors to yield information about the condition of that test object and determine whether a loss of material due to corrosion has occurred. A low-frequency eddy current inspection method can detect a loss of material in a metallic structure but is not always reliable. It often requires the use of dual frequency methods and signal mixing to detect corrosion.
Canadian Patent 2,102,647 by John H. Flora et al is directed to detecting defects in a metal component using a low frequency eddy current technique. John H. Flora et al uses an excition coil wound on a yoke and a pair of magnetic flux sensors differentially connected with respect to each other in an area under the yoke. The differential connection will result in the cancellation of common signals detected by the sensors, those which would be generated by the coil, but allow the detection of other signals generated by eddy currents in the metal component. The yoke is then placed near the metal component and a low frequency alternating current applied to the coil to generate eddy currents in the metallic component, which currents are detected by the sensors. The yoke is moved along the surface to scan for defects by changes in the generated eddy currents at defect locations.
U.S. Pat. No. 4,843,319 by Pedro F. Lars and U.S. Pat. No. 4,843,320 by Brian R. Spies are directed to corrosion detection where a transmitting antenna coil is placed next to a metal container, in this case a pipe with layer of insulation on it, and applying a train of pulses to that transmitting coil. The pulses are shaped so the coil is energized for a sufficient period of time to stabilize the magnitude of the field, with no eddy current then being generated, and then de-energizes abruptly to generate eddy currents in the metal which are detected by a receiving coil. Those eddy currents decay and are gradually dissipated within the metal with the rate of diffusion being dependent on the conductivity and thickness of the metal. The decay of those eddy currents is detected by a receiving coil and used to determine if defects in the metal exist such as caused by corrosion and a resulting change in thickness of the metal. However, errors in responses will occur due to variations in distance between the antenna and the metal wall of the container. Pedro F. Lara discusses some methods for correcting those errors in responses. The pulses used in these US Patents operate in the time domain rather than in a frequency domain manner as used in Canadian Patent 2,102,647. In the time domain, the information needed to probe a conductor wall for reasonably accurate detection can be obtained with one transmitted pulse. Each pulse contains an infinite number of frequencies. In frequency domain methods, however, only a few frequencies are used to probe a conductor wall which results in a limited amount of information from which the wall thickness is to be determined.
U.S. Pat. No. 6,037,768 by John C. Moulder et al describes another pulsed eddy current (PEC) apparatus to detect corrosion in metal structures such as aircraft lapjoints. John C. Moulder et al describes a calibration of the PEC instrument before the inspection with a reference structure that the user knows to be flaw-free. The PEC probe, once calibrated, scans in serpentine fashion a selected fashion area under computer and motor driven control. John C. Moulder et al indicates in lines 38 to 44 of column 4 that an air gap between the probe and lapjoint is known as xe2x80x9clift-offxe2x80x9d and that ideally, lift-off remains constant at 0.007 of an inch during a scan since the probe has a constant built-in wear surface. However, possible irregularities in a lapjoint surface may result in greater lift-off with a possibility of obtaining anomalous inspecting result. The user, during a scan is, however, able to filter from the display known conditions such as the existence of fasteners and airgaps and excessive probe lift-off.
Prior art methods of detecting corrosion in aircraft lap splices multi-layer metallic structures have proven inadequate. The detection of corrosion by either ultrasonic or eddy current techniques is not inherently difficult, but, there are problems with the identification and characterization of that corrosion due to the complexity of multi-layer structures. To quantify the thinning in multi-layer structures, it is required to determine in which layer corrosion has occurred. Ultrasounds, for instance, will not easily penetrate beyond the first layer. Eddy current techniques, on the other hand, have the ability to perform multi-layer inspections without requiring a mechanical bond. Notwithstanding these limitations, most operators have elected to conduct visual inspections followed by low-frequency eddy current inspections to detect corrosion in aircraft lap splices. This approach reduces the number of false indications but it is not capable of isolating corrosion below 10% thinning in the first layer. Further, second and third layer corrosion may also progress to much greater amounts of thinning before they are finally detected by this approach.
It is an object of the present invention to provide a pulsed eddy current method for detecting defects in a metallic structure and allow a quantitative evaluation of any defects detected.
A method for detecting defects in a metallic structure, according to one embodiment of the present invention, comprises locating a transducer at a first distance from said metallic structure at one area that lacks any significant defects in the structure, activating said transducer with a square wave voltage controlled excitation to generate eddy currents in the structure and then sensing, with at least one magnetic flux sensor, time-varying magnetic fields generated by the transducer and said eddy currents, signals obtained from said at least one sensor being recorded, this process being repeated to obtain at least one other recorded signal that is obtained with the transducer being locating in the same location but at a different lift-off distance from said one area, determining where the recorded signals cross to establish a Lift-off Point of Intersection at a point in time, placing said transducer at other areas of said structure which are to be tested for defects, activating said transducer with similar voltage-controlled excitation as applied at said one area, then obtaining and recording signals sensed from the time-varying magnetic fields generated by the transducer and eddy currents in a similar manner as at said one area, comparing the recorded signals amplitudes which are obtained at said other areas at said point in time with those of signals obtained at said one area with differences in signal amplitude providing indications of any defects present at areas being tested.